Some notions regarding morphology adapted from Suzette Haden Elgin from her book What is Linguistics?

Definitions
A morpheme is a meaninful sequence in a language which cannot be divided into any smaller meaningful units; it may or may not be a word. Thus, elephant is both a word & a morpheme; books contains the morpheme (& word) book and an additional morpheme, the {—s} that marks the plural, which is not an independent word. In Kumeyaay, an American Indian language of California, commands are indicated by a morpheme attached to the verb. The Kumeyaay word menak has two morphemes—{me—} (‘you’) & {—nak} (‘sit down’)—& is a statement. Kenak is made up of {ke—}, the imperative marker, & the same {—nak}, & is a command meaning ‘Sit down!’.

A full set of grammatical forms for a verb is called a conjugation, and all sets of such forms, whatever their part of speech, are known as paradigms.

Inflectional categories
In Navajo it is not sufficient to state simply that someone has gone somewhere. The form of the verb must specify whether that person has gone & returned, gone & arrived & gone no farther, gone & arrived & continued on past the original destination, gone & is still on the way to the destination, & so on. The degree of specificity demanded has as one result the fact that there are more than 3000 spearate forms for the Navajo verb go.
The perception of time & duration may also vary from one language to another. For instance in Navajo, the verb sidá, which is translated into English as ‘she, he, it is sitting’ or ‘the two of them are sitting’, is a Navajo past tense form. From the Navajo point of view, to be sitting is a completed action by one who has stopped moving, & hence is an event in the past.

Underlying Representations (=base forms)
A major problem for linguists is that of deciding upon the correct underlying representation of a morpheme. If we consider the 3 negative prefix allmorphs composed of /i/ & a following nasal, we run into this question: Is /in—/, /im—/, or /iN—/ the underlying form from which the other two allomorphs are derived?
The data shows that when no consonant follows the prefix it is /in—/; therefore, this can be chosen as the basic underlying form, & the variants /im—/ & /iN—/ are derived by the rule assimilating /n/ to the place of articulation of the following consonant.

The application of phonological rules to morphological sequences often obscures their underlying structure & makes them appear to be "irregular” Look at the following Navajo examples:

Naashnish. I am working.
Nanilnish. You are working.
Naalnish. She is working. (Or he, it, they two are working.)

There are two phonological rules of Navajo which apply to this set of forms. One is a rule that deletes the phoneme /l/ whenever it occurs between two consonants. The other rule, which must take morphological information into account, applies to the Navajo morpheme {na—}, a verb prefex meaning roughly ‘around’, & states that its vowel becomes long whenever there is no other vowel present in the verb between {na—} & the verb stem. This rule does not apply to just any vowel /a/ in Navajo—only to the one appearing in the morpheme {na—}.

Tense
Tense can be looked upon in two ways. In semantic terms, it refers to the time of the predicate in a sentence, while in morphology it refers to the manner in which a verb is marked to indicate that time. Ideally there would be a perfect match between the two, so that the attachment of a morpheme meaning present tense would always & without exception mean that the predicate involved was one in present time. In English, we do not have this sort of ideal correspondence, & the so-called present tense forms of our verbs in isolation cannot be relied upon to refer to present time at all.